Hello, This is mltony, I am working on NVDA add-on called phonetic punctuation:
https://github.com/mltony/nvda-phonetic-punctuation
IBM TTS driver doesn't seem to handle callback commands in python 3 version correctly in some cases. Here are steps to reproduce:

1. Install phonetic punctuation add-on:
   https://github.com/mltony/nvda-phonetic-punctuation/releases/download/v0.2dev/phoneticPunctuation-0.2dev.nvda-addon
   Note that it requires NVDA 2019.3 alpha.
2. Speak the following phrase:
   Test test!!!

Expected behavior: Ding-ding-ding sounds should start playing after "test test"
Actual behavior: ding-ding sounds play at the same time "test test" utterance is spoken.

Phonetic punctuation converts this utterance into:

["Test test", <CallBackCommand that actually plays those ding sounds>, <BreakCommand for the duration of the sound>]

SO I suspect that your driver triggers callback sooner then the utterance has been fully spoken.

With other synthesizers phonetic punctuation works correctly; I tested with espeak, SAPI, OneCore and the other version of eloquence.

There is also another small problem, it seems like the duration of the pause in break command must be multiplied by some coefficient, that seem to be equal to 3. If you try to speak this phrase with phonetic punctuation:
!!!!!!!!Test
You will hear the word "test" much sooner than the dings end, because of that problem.